I love knuckle bones for beef broth and a full chicken carcass with skin and bits of meat for chicken broth. I also recommend a good "glug" of apple cider vinegar in the pot to bring out the minerals in the bones. I brew it in the crock pot for 24 hours and I get a nice gelatin.

I use beef shanks, which are great because it has all the bone, the marrow, the connective tissue, and the meat for flavor. Marin Sun and Prather Ranch both have grass fed beef shanks and beef bones, but Marin Sun will have more specialty cuts, such as knuckle, organs, etc.

You could also try your nearest Whole Foods market, which will probably sell beef bones for around $3.99/lb or around $6/lb for shanks. Sometimes vendors will have shanks labeled as Osso Bucco.